But, as the Moon goes round the ecliptic in 27 days, 8 hours, she is only visible during 13 days and 16 hours, and as long invisible to each pole by turns. All the planets likewise rise and set to the polar regions because their orbits are cut obliquely in halves by the horizon of the poles. When the Sun arrives at the sign Aries, which is on the twentieth of March, he is just rising to an observer on the north pole, and setting to another on the south pole.* [Plate 6th, figure 2d, spring and autumn.] From the Equator, he rises higher and higher in every apparent diurnal revolution, till he comes to the highest point of the ecliptic on the 21st of June, and then he is at his greatest altitude, which is 23 degrees and 28 minutes; equal to his greatest north declination, and from thence he seems gradually to descend in every apparent circumvolution till he sets at the sign Libra, on the 23d September, and then he goes to exhibit the same appearances at the south pole for the other half of the year. Hence the Sun's apparent motion round the Earth is not in parallel circles, but in spirals, such as may be represented by a thread wound round a globe from one tropic to the other. If the observer be any where on the terrestrial equator, he is in the plane of the celestial equator or under the equinox, and the axis of the earth is coincident with the plane of his horizon extended to the north and south poles of the Heavens. As the earth performs her diurnal revolution on her axis from west to east, the whole heavens seem to turn round the contrary way. It is therefore plain that the observer at the equator has the celestial poles constantly in his horizon, and that his horizon cuts the diurnal paths of all the celestial bodies perpendicularly, and in halves. Therefore, the Sun, planets and stars rise every day, and ascend perpendicularly above the horizon for six hours, and passing over the meridian, descend in the same manner, for the six hours follow * It is therefore evident when the Sun is on the Equator, an observer placed at each pole, sees about one half of the Sun above the horizon, and likewise an observer at the Equator discovers both poles in the horizon. ing, then set in the horizon and continue 12 hours below it; consequently the days and nights are equally long throughout the year. Thus we find, that to an observer at either of the poles, one half of the sky is always visible, and the other half never seen; but to an observer on the equator, the whole sky is seen every 24 hours. From the preceding observations, it is evident that as the Sun advances from the equator to the tropic of Cancer, the days continually lengthen, and the nights shorten in the northern hemisphere, and the contrary in the southern; and when the Sun descends from the equator to the tropic of Capricorn, the days continually lengthen in the southern hemisphere, and the nights shorten; and the contrary in the northern. The earth's orbit being elliptical, and the Sun constantly keeping in its lower focus, which is one million three hundred and seventy-seven thousand miles from the middle point of the longer axis, the earth comes twice that distance, or 2,754,000 miles nearer the Sun* at one time of the year than at another; for the Sun appearing under a larger anglet in our winter than summer, proves that the earth is nearer the Sun in winter than in summer. The Sun is about 7 days longer in the northern hemisphere than in the southern in every year; and as the earth approaches nearer to the Sun, its motion is accelerated, and therefore goes over an equal space in less time, and as the earth recedes from the Sun, its motion is retarded in the same ratio that it was accelerated when in the southern hemisphere, and consequently requires a longer time to pass over an equal space. But here a question actually arises; why have we not the hottest weather when the earth is nearest to the Sun? In answer, it must be observed, that the eccentricity of the earth's orbit bears no greater proportion to the earth's mean distance from the Sun, than seventeen * The Sun is nearest to the Earth when he is on the tropic of Capricorn; farthest from it when he is on the tropic of Cancer. + The nearer an object is to the eye, the larger it appears, and under the greater angle it is seen. bears to a thousand, and therefore this small difference of distance cannot occasion any great difference of heat or cold. But the principal cause of this difference is, that in winter, the same rays fall so obliquely upon us, that any given number of them is spread over a much greater portion of the earth's surface which we inhabit, and therefore each point must then have less rays than in summer. [See plate 5th, fig. 6.] Also there comes a greater degree of cold in the long winter nights, than there than can return of heat in so short days, and on both these accounts, the cold must increase. But in the summer season, the Sun's rays fall more perpendicularly upon us, and therefore come with greater force, and in greater numbers, on the same place, and by their long continuance, a much greater degree of heat is imparted by day than can fly off by night. It is for this reason that we have a greater degree of heat in the month of September, than in the month of March; the Sun being on the equator in both these months, and consequently equally distant from the earth. Those parts which are once heated, retain the heat for some time, which, with the additional heat daily imparted, makes it continue to increase, though the Sun declines towards the south, and this is the reason why we have greater heat in July than in June. Also, we know that the weather is generally warmer at 2 o'clock in the afternoon, when the Sun has gone towards the west, than at noon, when he is on the meridian. Likewise those places which are well cooled, require time to be heated again, for the Sun's rays do not heat even the surface of any body till they have been some time upon it, and therefore January is generally colder than December; although the Sun has withdrawn from the winter tropic, and begins to dart his beams more perpendicularly upon us. It was formerly the opinion of Philosophers, that the Sun was an immense mass of flame, and consequently the nearer we approached, the greater must be the heat. It is stated that the heat of the planet Mercury is seven times as great as ours, judging from his nearness to the Sun, and likewise that the cold at the planets Jupiter, Saturn and Herschel, must be extreme, because they are placed at so great a distance from that luminary. It is well known, that near the equator, the tops of the highest mountains are covered with perpetual snow, and that in a less distance than three miles above the surface of the earth, we come to the region of perpetual congelation, where neither ice nor snow would ever melt, although nearer the Sun than in the plane below. Therefore, the distance from the Sun is not the real cause of heat or cold. Dark spots have been seen upon the Sun's disk, from which it is generally concluded that the body of the Sun is dark and opaque, surrounded by a luminous atmosphere, which darts its rays with immense velocity, and by some chymical operation, performed in their passage through the atmosphere with which this earth is encircled, conveys to us the sensation of heat. The solar observations of Dr. Wilson, first suggested the opinion, that the Sun was an opaque and solid body, surrounded with a luminous atmosphere, and the telescopes of Dr. Herschel have tended still farther to establish this opinion. The latter of these astronomers, therefore imagined, that the functions of the Sun, as the source of light, might be performed by the agency of the external atmosphere, while the solar nucleus was reserved, and fitted for the reception of inhabitants. That the Sun may at the same time be the source of light and heat, and yet capable of supporting animal life is one of those conclusions, which we are fond of admitting without hesitation, and to cherish with peculiar complacency. The mind is filled with admiration of the wisdom of that Benign Benefactor, and swells with the most sublime emotions, when it conceives that apparently the most inaccessible regions of creation are peopled with animated beings, and, that while the Sun is the fountain of the most destructive of all the elements, it is at the same time the abode of life and plenty. When the invention of the telescope enabled Astronomers to detect the striking resemblances between the different planets of the system, it was natural to conclude, that, as they were composed of similar materials, as they revolved around the same centre, and were enlightened by similar Moons, they were all intended by their wise Creator to be the region in which he chose to dispense the blessings of existence and intelligence to various orders of animated beings. The human mind cheerfully embraced this sublime view of creation, and guided by the principle, that nothing was made in vain; man extended his views to the remote corners of space, and perceived in every star that sparkles in the sky, the centre of a magnificent system of bodies, teeming with life and happiness, and displaying fresh instances of the power and beneficence of that Being who rolled such stupendous orbs from his creating hand. Having thus traversed the illimitable regions of space, and considering every world which rolls in the immense void as the scene on which the Almighty has exhibited his perfections, the mind, unable to command a wider range, rests in satisfaction on the faithful analogies which it has pursued. INTERROGATIONS FOR SECTION FOURTH. How much of the Heavens does a spectator see placed at the north pole of the earth? What part if placed at the south pole? What part if seen at the equator? When placed at the north pole, how far south in the Heavens would his vision extend? If at the south pole, how far north would his vision extend? Does any part of the ecliptic, or orbit, which the Sun seems to describe once a year, appear above the horizon of the north pole? What signs of the ecliptic appear above the horizon of the north pole? Ans.-Aries, Taurus, Gemini, Cancer, Leo and Virgo; these are called northern signs. What signs appear above the horizon of the south pole? |